The present invention relates in general to systems for measuring signal parameters and in particular to a method and apparatus for measuring jitter in a periodic signal.
Periodic clocks are often used for timing purposes. For example a periodic clock signal may control a digitizer so that it periodically digitizes an input signal, thereby producing a data sequence representing the input signal as a function of time. In such an application the clock signal frequency should be substantially constant because frequency variation in the clock signal will cause the digitizer to digitize the input signal at unexpected times. In such case the data sequence provides a distorted representation of the input signal magnitude as a function of time. "Jitter" in a periodic signal is defined as a difference between a time at which an event in the signal (such as a zero crossing or a peak) actually occurs and an expected time of occurrence of the event if the frequency of the signal was perfectly stable. Thus the instantaneous jitter of a clock signal is a measure of its timing error at any given time. To evaluate the frequency stability of a clock signal source, it would be helpful to provide a quantitative measurement of its jitter as a function of time.
Jitter in a periodic signal can sometimes be detected by providing the signal as the vertical input to an oscilloscope having a horizontal sweep repetitively triggered by an event in the signal itself. Large amounts of jitter cause the resulting waveform display to be fuzzy. However, such a method of detecting jitter is usually not very effective when a periodic signal has only small amounts of jitter, and such method does not provide a quantitative measurement of jitter in a periodic signal as a function of time.